Bangkok, Thailand
Abstract
The 238.5-km Yadana gas pipeline of the Petroleum Authority of Thailand (PTT) passed a 50-km strip of forest in which 20-m width was allowed for use during construction, amounting to an area of around 96 ha. Bare soil results from the construction was immediately exposed to erosion. Without any preventive measure, especially for the Slope Complex Soil Series in high mountain area, erosion rate may increase by 100 times.
To minimize environmental impact during construction and to achieve maximum effectiveness in rehabilitation, a construction strategy was formulated to create permanent soil erosion control structure with physical erosion design approach, i.e. soft plugs, diversion berms, and drainage system, combined with re-vegetation and agronomic erosion control methodology along the pipeline right of way. Vetiver grass was specifically introduced in high-risked erosion areas.
Erosion rating was calculated and classified into low risk (E0), moderate risk (E1), high risk (E2) and very high risk (E3). The mitigation measures at each area can be elaborated with the following details: E1 areas - building permanent soil erosion control structure, re-vegetating with three kinds of leguminous covers, sowing of various grass species and immediate re-vegetation before the first rainy season; and E2 and E3 areas - building permanent soil erosion control structure including soft plugs and berms as well as vetiver grass planting, sowing of various grass and leguminous species before the first rainy season and re-vegetation before the second one.
Vetiver grass was planted in the E2 and E3 areas along the contour line, 10-15 cm apart, along the same contour furrow while hedgerows were spaced about 1-1.5 m, amounting to 1.5 million slips used in this project.
In the E3 area with over 35o slopes, berm construction may not effectively prevent erosion during the first rainy season; therefore, jute sacks filled with a mixture of soil and grass seeds were arranged in ladder-pattern, pegged with live stakes. Grass and legume seeds were sowed. Sacks were holed and planted with vetiver slips so that their roots grew into the soil while sacks naturally decayed. Those laddered jute sacks naturally formed permanent soil ladders covered with vetiver grass and other vegetation.
The vetiver grass system when utilized in conjunction with physical structures will play a significant role in erosion control and slope stabilization.
The pipeline laying in the Yadana Gas Pipeline Project was necessitated to pass a distance of 50 km in the forest area with the construction right-of-way (R/W) of 20 m. Due to construction characteristics, strip surface clearing and construction resulted in bare soil after pipeline installation, thereby exposed to immediate soil erosion impact. The most effective application of rehabilitation measures must be implemented, based on environmental evaluation, engineering design / construction, and re-vegetation / agronomic control. Vetiver grass was thus introduced in the high-risked erosion areas.
Procedures and methods in integrating construction-engineering practices with botanical knowledge are summarized in the flow chart and the content of this document would subsequently follow the chart shown in Fig. 1.1.
2. PROJECT DESCRIPTION
During 1986 to early 1997, Thailand's commercial energy consumption, i.e. oil, coal/ lignite, natural gas and hydropower recorded a continuous growth, a three-fold rise from 300,000 barrels per day (crude oil equivalent) to 900,000-1,000,000 barrels per day. As indigenous energy sources were quite limited, able to be produced not over 30% of the demand, while the remaining must be imported, 60% of which is oil, in case of price fluctuation in global market, impact can be perceived. Therefore, additional procurement of natural gas was urged especially for power generation. More importantly, natural gas among petroleum fuels is cleaner. The Yadana Gas Pipeline Project was thus originated.
2.1 Yadana Gas Pipeline Project
The Petroleum Authority of Thailand (PTT) constructed the Yadana Gas Pipeline Project from the Thailand-Myanmar border at Ban I Tong, Thong Pha Phum district, Kanchanaburi province to transmit natural gas to fuel the Ratchaburi Power Plant of the Electricity Generating Authority of Thailand (EGAT) as well as Independent Power Producers (IPP). The 238.5-km pipeline passed a 50-km strip of forest in which 20-m width was allowed for construction space, amounting to an area of around 96 ha.
The construction of this 105-cm diameter pipeline, like other pipeline construction, was involved with digging of 2.50-m wide and 2.50-m deep trench. Pipelines were welded, strung together and installed. In the forest area, delivery of construction materials restricted within pipeline R/W and existing original mining roads in which necessary upgrading was done. However, during operation phase, along the pipeline route in forest areas, no permanent road was built.
The overall gas transmission facilities consist of 12 block valve stations located along the pipeline and one Operation and Maintenance Center (O & M) in Ratchaburi province. The system is monitored and controlled via the Supervisory Control and Data Acquisition System (SCADA) from both O & M in Ratchaburi and the Operations Center in Chon Buri province. To control the pipeline, on-line information from the pipeline inlet and outlet, i.e. pressure, temperature, density and volumetric flow are monitored. On-line pipeline integrity monitoring software in the SCADA system performs overall pipeline mass balance, pipeline hydraulics and rate of change pressure calculations, including the detection for leakage or any abnormal operation.
2.3 Standards of Engineering Design and Equipment
Pipeline transmission is globally accepted as highly safe as pipes are fabricated with special steel, buried underground, and virtually separated from mass transportation system.
(space for Fig. 1.1)
2.4 Overall Project Implementation Schedule
The construction at site was started on 24 March 1997 and completed on 30 June 1998, which finished on schedule.
| The implementation list | 1992 | 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | |
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Source: The Yadana Gas Pipeline Project
Along the Thailand-Myanmar border, the dominant land use type is forest area, where the pipeline route has to pass. It is necessary to study the alternative routes and choose the one with minimum environmental impact.
PTT studied a route that pipeline may be installed along the R/W of the Highway 3272 (Thong Pha Phum - Ban I Tong). However, certain constraints were found and implementation was dropped:
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: | The gas pipeline must be installed along side slope for a distance of 30 km. The area was thus liable to subsiding. |
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: | With over 200 curves of the road some are sharp curves, the equipment for internal inspection of the pipeline may not be able to run smoothly. |
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: | The road was 6-8 m wide and was not sufficient for construction space. Mountain slopes must be cut in for another 8-m wide, leading to greater environmental damage and erosion. |
3.2 Alternative Routes and Selection
Three potential alternative routes located in forest area were investigated. Three possible routes were identified as Alternative 1, 2 and 3 as shown in Fig. 3.2. The most appropriate route was selected based on the level of environmental impact and was subsequently studied for more details.
The gas pipeline route must pass national reserve forest (conservation forest area), national park and watershed class 1A areas. The following six factors were taken into consideration for environmental impact study: (i) forest resource, (ii) wildlife, (iii) socio-economic and community, (iv) hydrology, water quality and aquatic ecology, (v) local transportation, and (vi) aesthetic and cultural value.
| The study, the details of which appeared in the Environmental Impact Assessment (EIA), revealed that the Alternative 1 was the most appropriate, based on its least environmental impact compared to the other alternatives. In addition, it would pass the shortest distance of national reserve forest (conservation forest area) and watershed class 1A areas. |
4. ENVIRONMENTAL EVALUATION
The EIA of the Yadana Gas Pipeline Project was started in 1995. The objectives of the study are as follows:
(space for Fig. 3.2)
Environmental evaluation was undertaken on two topics, i.e. existing environmental conditions and environmental impact assessment due to the construction. The former was used as references and baseline data while the latter was beneficial for the formulation of mitigation measures. The study on both topics covered the same 13 environmental issues, the details of which appeared in the EIA. Significant issues of the EIA due to the construction are as follows:
Erosion is strictly significant as the construction procedures include the clearing of 20-m wide area along the construction R/W. After the backfill, bare soil will directly affect erosion and immediately as soon as the rainy season arrives. Therefore, impact assessment must be undertaken on `worst case', i.e. from Kilometer Post (KP) 0 to KP 27 that would be most affected by erosion. The assumption was made in the width of 50-m Right of Way after the construction, the soil was exposed without any mitigation measure. From that `worst case' assumption, it was found that the overall maximum percentage of sediment to be washed from cleared R/W is about 1.285 % of the total sediment in Khwae Noi River close to the project area. Based on the impact resulting from the `worst case' scenario assumption, the potential erosion affected by the construction of the Project can be minimized by indispensable mitigation measures.
(3) Water Quality: The impact on water quality, especially in streams and rivers due to the construction and related activities, is a result of:
(4) Aquatic Ecology: The impact on aquatic ecology was fairly low because the construction in forest area especially in watershed area is done in the dry season only when the water flow is minimized and several streams were seasonally dry.
(5) Soil: Soil was directly affected due to construction characteristics. After the back-filling, the soil was bare and erosion immediately occurred in the rainy season, especially from KP 0 to KP 50 in the high mountain area and the soil was in series of Tha Yang, Lat Ya, Mae Rim, and Slope Complex. With vegetation (prior to the construction), such soil series had the erosion rate of around 16-21 t/rai/year; however, as bare soil without vegetation, high erosion would be materialized with a rise to 69-2,000 t/rai/year. This would yield further impact, i.e. the loss of soil nutrients, water quality affecting community and especially the pipe itself.
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(10) Compensation: The compensation was fairly implemented in accordance with laws and regulations supervised by the Pricing Reconciliation Subcommittee chaired by the Governor of certain provinces. PTT / the Contractor have to follow their value specifications.
(11) Socio-economic: Local people / communities were positive with the Project as it would help with local economy during the construction and be beneficial to the country on the whole. However, negatively, they would be affected by various activities during the construction and were worried about the pipeline safety.
(12) Public Health: The impact was imminent during the construction period such as dust, noise and traffic, which would last about 2-3 months in each area. Safety measures and precaution must be held during construction in the R/W with hard bed rock that required blasting.
(13) Archaeology and Historical Value: The nearest archaeological and historical site, according to the study, is Wang Krachae Cave, 0.5 km away from the pipeline. The impact was not anticipated.
The 13 topics of the EIA due to the construction led to the formulation of detailed mitigation measures to accommodate the impact, as summarized below, of which details appeared in the EIA:
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Among the 13 environmental impact issues, soil erosion is prioritized as the utmost immediate impact due to the following reasons:
Measures to minimize impact from soil erosion are the most indispensable. Therefore, both construction strategies/techniques and rehabilitation have been taken into account. A special focus had been put on the selection of soil-holding vegetation in the areas with high risks of erosion.
With a principal objective to effectively prevent soil erosion after pipeline installation, ranked as utmost immediate impact to the environment and pipe itself, a construction strategy was formulated with the construction of permanent physical erosion control structure as follows:
On completion of back-filling, a crown 0.5 to 1.0 m. high shall be left over the centerline of the pipe, which results in speeding up of water runoff from centerline R/W.
Berms, a temporary ridge of compacted native soil constructed at intervals across the R/W with sufficient grade to provide drainage, are a form of surface drainage control device to deflect surface water off the R/W. It is a major erosion control device for undulating and steep slopes.
Table 6.1: Minimum soft plug and diversion berm spacing (m)
| Soil erosion potential | |||
| Slope | High (fine sand, silts) |
Moderate (clays, coarse sand) |
Low (gravel, exposed bed rock) |
| Gentle (under 5%) | 45 | 60 | - |
| Moderate (5 - 10%) | 30 | 45 | 60 |
| Steep (over 10%) | 305 % Grade |
% Grade |
% Grade |
Remarks: 305 = a constant slope factor
Source: Construction Procedures of the Yadana Gas Pipeline Project
Berm direction and orientation including pattern and spacing of the berms will be influenced by local topography, soils, adjacent vegetation cover, and drainage conditions, and shall be determined on site. Typical berm orientations are diagonal diversion berm, herringbone diversion berm, and horseshoe diversion berm.
After the diversion berm completion, permanent drainage system, i.e. riprap, pipe drain, drain channels shall be installed on the R/W immediately. Then, the “site” is reinstated back to the original topography and conditions, maintaining the slopes and general terrain characteristics. Surface runoff, especially, is not concentrated on a filled area where surface scouring and gully erosion can easily occur.
Special and state-of-the-art techniques were chosen for the construction to minimize environmental impact.
Despite the utilization of special techniques, conventional system such as Open Cut must be carried out under stringent mitigation measures, as well as in several areas with hard rocks, blasting was unavoidable constrained by time limit.
(space for Fig. 6.1)
With the characteristics of Slope Complex Soil Series, after the construction with bare soil, the EIA report indicated that the high erosion was 100 times increased, though in terms of engineering construction, permanent physical erosion control structure had been installed. An important one was diversion berms which served to deflect water off the R/W. However, without vegetation, these structures would be eventually perished by water runoff. Despite the presence of vegetation, in the high-erosion area, those ordinary shallow-root vegetation species were unable to resist water runoff. Therefore, in areas with high-risk erosion, vetiver grass was introduced to stabilize the area and berms which ultimately became natural permanent stabilized structure.
Vegetation must be carried out instantly after the reinstatement, and vetiver grass must be planted according to technical principles.
A mixture of vegetation was planted along the R/W:
Grasses: Brachiaria ruziziensis, Panicum maximum
Legumes: Centrosema pubescens, Pueraria phaseloides, Calopogonium muconoides
This would allow for better chances of vegetation to grow and cover the soil in diversified area characteristics.
Grow 1.5 million vetiver grass slips in areas with high risk of erosion: With remarkable characteristics given below, vetiver grass is appropriately qualified to create Permanent Physical Erosion Control Structure into Natural Permanent Stabilized Structure:
The above characteristics of vetiver grass have been continuously disseminated through the study and demonstration of various Royal Projects in accordance with His Majesty the King's initiatives. Vetiver grass was thus selected and applied in high-risked erosion areas of the Yadana Gas Pipeline Project as follows :
The implementation results of planting vetiver grass confirmed its suitability in preventing soil erosion, which will be publicized more extensively.
Plant 61,250 perennial trees at the distance of 3 m from pipeline center: Perennial (21 species) and fruit trees (7 species):
Various techniques were introduced to help with the growth of vegetation and enhance the stability of physical erosion control structure.
Construction strategy that combines the use of physical permanent erosion control structure with agronomic and re-vegetation erosion control can be summarized as follows:
8.1 Methodology
Erosion rating of a particular stretch of the R/W is classified according to the rate of possible soil loss. The soil loss is calculated under the worst conditions: i.e. loosened topsoil, bare and with no protection or measures. The amount of soil losses is calculated by the Universal Soil Loss Equation. The classification for each rating E0 to E3 is shown in Table 8.1.
| Soil Loss (t / rai* / yr.) |
Rating Code | Erosion Rating Risk |
| 0 – 16 | E0 | Low Risk |
| 17 – 64 | E1 | Moderate Risk |
| 65 – 112 | E2 | High Risk |
| 113 above | E3 | Very High Risk |
* 1 ha = 6.25 rai
8.2 Application
Permanent physical erosion control structure, and agronomic and re-vegetation erosion control are appropriately applied to each erosion rating risk. In the areas classified as high risks in particular, vetiver grass was applied as tabulated in Table 2.
Table 8.2: Physical permanent erosion control structure combined with agronomic and re-vegetation erosion control
| Areas / condition |
Grass + legume |
Tree | Vetiver grass | Soft plugs & berm and drainage system |
Special technique |
| E0 | - | - | - | ||
| E1 | - | - | - | ||
| E2 | - | ||||
| E3 |
Remarks: In E0 areas, trees may not be planted
The 50-km R/W in the forest area was classified into E0, E1, E2 and E3 areas, of which rehabilitation program was applied according to the application. Above all, the mitigation measures must be stringently observed. The performance evaluation must be obtained after the first and second rainy seasons.
9.1 Implementation
The physical erosion control structure must be finished by June at the latest, after which the re-vegetation and agronomic erosion control must be immediately implemented. The results of its implementation according to the application along the 50-km R/W consist of six areas for E3, and 44 areas for E2, and the rest are E1 and E0. Those overall 50-km implemented results are plotted in Fig. 9.1.
9.2 Evaluation
Rehabilitation performance would be taken into consideration: how much and how quick the R/W in forestry areas has been recovered back to its original state.
Evaluation Concept: The universal soil loss equation was used in evaluating soil loss by considering multiplication results of cropping management factor, or C factor, i.e. physical erosion control structure, and erosion control practice, or P factor, i.e. agronomic and re-vegetation erosion control, while other param (rainfall erosion index, soil erodibility factor, slope length factor, slope steepness factor) are treated as constant terms. Multiplication results of C and P factors are index of rehabilitation performance.
Evaluation Methodology: Direct examination at site along the R/W was undertaken after the first and second rainy seasons. On the other hand, the average C and P factors of each KP would be evaluated. Normally, the C and P factors are ranged between 1 and near zero. The more multiplication results of C and P factors are near zero, the less soil erosion rating will be. In other words, the areas are reinstated back to pre-construction conditions. This indicates better rehabilitation performance each year.
Initially, after pipeline installation, the soil was bare, berms were not built yet and without re-vegetation; P and C factors were equal to 1.0. However, when berms were in place with necessary drainage system and when vetiver grass, vegetation and perennial trees started to grow, the P and C factors gradually decreased. Characteristics of the P factor are slightly different from those of the C factor. That is, the P factor in the first year when the physical erosion control structure was just finished was slightly better than subsequent years by about 5-15%. The C factor would be better in accordance with the growth rate of the re-vegetation.
The evaluation of multiplication results of the C and P factors of each KP at the end of the first rainy season and the second rainy season was plotted in comparison with timing as shown in Fig.10.1, of which significance is as follows:
10.1 Overall Performance
(space for Fig. 10.1)
10.2 Significant Roles of Vetiver Grass
In using vetiver grass in high erosion area to prevent erosion and to conserve soil and water applied in pipeline route, the following significant issues were found:
There were some points in which erosion occurred where vetiver grass had been grown, a result of improper and incomplete implementation of permanent erosion control structure. For example, berms were not continuous with the stabilized area, too steep degrees of berms. However, with proper execution and planting methods of vetiver grass, the physical erosion control structure would be naturally made into erosion control area within the first rainy season and would be even more stable after the second one.
The performance of utilizing vetiver grass in preventing soil erosion and creating soil moisture of this Project will be continuously monitored and disseminated to the public so that its usage can be further extended in both engineering and agricultural aspects even more.
11. FURTHER IMPLEMENTATION IN PTT'S REFORESTATION PROJECT
In the Reforestation Project of PTT, vetiver grass has been planted in appropriate areas. People around the Project are encouraged to learn how to use vetiver grass in conserving soil and water. The 9,972-Rai Reforestation Project of PTT is a part of the Kui Buri Forest Conservation and Rehabilitation Project under His Majesty the King's Initiative. Vetiver grass has been incorporated into the Project in the following manners:
To encourage people to use vetiver grass in their own lands is beneficial to agriculturists themselves. Results achieved may be quite slow but are considerably useful in the long run, as it can reduce agricultural costs in accordance with His Majesty the King's concept granted on 29 May 1998:
“Conserve land by using vetiver grass because its root system is long and can sustain water to create moisture and improve soil.”